Thin film metallization of ceramics is important in at least four technologies. Firstly, in electronic packaging, metal lines are used in multilayer ceramic structures for electrical interconnects. Secondly, very thin metal films supported on ceramic substrates are used as catalysts, for example, in automobile catalytic converters. Thirdly, metal coated ceramic fibers are employed to fabricate metal matrix composites by liquid metal infiltration. Fourthly, devices that employ metal coated active or functional ceramic materials find application in optical, sensor and superconductor technologies.
In all these cases there is a need for providing thin continuous and/or patterned metal films on ceramics. In the case of electronic packaging, metal line density is limited because of the tendency of thin films to become discontinuous. In the case of catalysts, discontinuities in the film lowers efficiency. In the case of metal matrix composites fabricated from fibers, a discontinuous metal coating requires higher infiltration pressure on the liquid metal.
Kennefick, C. M., et al, Acta Metall. Vol. 37, No. 11, pp. 2947-2952 (1989) discuss the problem of the instability of thin metal films when heated to a temperature ranging from 25% of the melting point expressed in degrees Kelvin up to just below the melting point. The instability manifests in the fragmentation of the metal film into islands, hereinafter referred to as beading of the thin film. The same phenomenon is important in all of the applications described above.
The terms "thin metal films" and "thin metal coatings" are used herein to indicate thickness normal to the substrate of at least about 10 nm and less than 10 microns.